The invention is in the field of electronic reproduction technology and is directed to a method for white balance in color scanner devices, also referred to as color scanners, for point-by-point and line-by-line scanning of color originals with an optoelectronic color scanner element.
In a color scanner, the scan light coming from the color original is first resolved with dichroitic filters into the color components xe2x80x9credxe2x80x9d, xe2x80x9cgreenxe2x80x9d and xe2x80x9cbluexe2x80x9d and is supplied to the individual color channels, wherein the three color components of the scan light are then converted with optoelectronic transducers into the color signals for xe2x80x9credxe2x80x9d, xe2x80x9cgreenxe2x80x9d and xe2x80x9cbluexe2x80x9d.
The color signals are further-processed in signal editing stages following the optoelectronic transducers. The signal editing stages comprise a defined signal input range whose corner signal values are referred to as white level and as black level.
Before the beginning of scanning, the density scope of the color original to be scanned is adapted to the defined signal input range of the signal editing stages by a white balance of a color scanner. What is thereby achieved is that the scan light coming from the brightest location of the color original to be scanned, the white point, is converted in the optoelectronic transducer into a color signal value that corresponds to the white level.
Since the scan light coming from the brightest location of the color original varies from color original to color original and the sensitivity of the optoelectronic transducer is not constant over a longer time span, a corresponding white balance is implemented in practice before every scanning of an original, this being complicated since at least three optoelectronic transducers must be balanced.
DE-A-25 45 961 already discloses a method for the automatic white balance of scanners. In a calibration phase, the scanner element of a black/white scanner is positioned to the respective white point of the image original and the scan light coming from the targeted white point is converted into an actual image signal value in the optoelectronic transducer. The actual image signal value is compared in a control unit to a rated image signal value that corresponds to the white level. A control signal changes the sensitivity of the optoelectronic scanner and/or the gain of a following amplifier until the repetitive error is zero. The control signal value required therefor is stored for the duration of the originals"" scanning following the calibration phase. For white balance in color scanners, the control unit is expanded to the three color channels.
The known method has the disadvantage that a corresponding white point on the image original to be reproduced must always be targeted with the color scanner element in the white balance, this being time-consuming and, particularly given repetitions of the white balance, imprecise. Added thereto is that a bright image location suitable as a white point is often not present in a chromatic image original.
EP-A 0 281 659 discloses a further method for the white balance of scanners wherein the repeated approach of a white point with the color scanner element on an image original to be reproduced is avoided. For that purpose, a light attenuation factor is determined in the initial white balance by optoelectronic scanning of the white point. Given repetitions of the white balance, the scan light coming from the white point is simulated by the light of the light source attenuated according to the identified light attenuation factor without a repeated scanning of the white point in the image original, whereby the light attenuation is undertaken with a controlled iris diaphragm.
The known method is involved and is based on a color-neutral density simulation, which is not always established in practice, and can therefore occasionally lead to unsatisfactory results. In the known method, further, no density simulation is possible in the scanning of opaque originals since the iris diaphragm is needed for the correct setting of the depth of field.
It is therefore an object of the present invention to improve a method for the white balance in a color scanner device for the point-by-point and line-by-line, optoelectronic scanning of color originals such that a simple, cost-beneficial and color-neutral simulation of the optical densities of white points in the color originals and, thus, a high reproduction quality is achieved.
According to the present invention, a method is provided for white balance of an opto-electronic color scanner element of a color scanner device for pixel-by-pixel and line-by-line scanning of a color original illuminated by a scan light source for trichromatic splitting of scan light modulated with optical densities of scanned picture elements and limited by a scan diaphragm into color components for each color channel and for conversion of the color components into color signals with a light/voltage transducer unit in every color channel. A white level UWP is predetermined for each color channel. Optical densities DWP of three color components of a white point which is a brightest location of the color original are measured. Rated voltage values USOLL are calculated that the actual voltage value should reach given an optical density xe2x80x9czeroxe2x80x9d of the color original for the three color components from the measured optical densities DWP of the white point of the color original and from said predetermined white levels UWP according to the following equation:
USOLL=UWPxc3x9710 exp. [DWP]
scan light that represents the optical density xe2x80x9czeroxe2x80x9d of the color original is simulated with a light source. The scan light representing the optical density xe2x80x9czeroxe2x80x9d is converted with said light/voltage transducer into color signals as actual voltage values for the individual color components. The actual voltage values are compared to the rated voltage values USOLL. Gains of the light/voltage transducer in the three color channels are set and held constant for a duration of the scanning of the original such that the actual voltage values that are generated given the scan light representing the optical density xe2x80x9czeroxe2x80x9d of the color original are equal to the calculated rated voltage values USOLL, and as a result thereof, the color signals of the white point of the color original acquired in the later original scanning correspond to the predetermined white levels UWP.